Comprehensive Lecture Notes – Vaccines & Virology

Lecture Outline

• What is a vaccine?

• Historical development of vaccination

• Virus biology (structure, classification, replication)

• Immune-system interactions with viruses & vaccines

• How vaccines work; types of vaccines

• COVID-19 used as a running case study

• Learning outcomes

  • Describe viral structure, classification & replication

  • Explain vaccine-induced immunity

  • Outline vaccine development stages (pre-clinical

    ➔ clinical)

  • Differentiate vaccine types & their immune mechanisms

Definitions & Core Concepts

• Vaccine = “product that stimulates a person’s immune system to produce immunity to a specific disease, thus protecting them from future infection.”

  • Practical phrasing: a harmless form or part of a pathogen (e.g., a viral protein or inactivated virus) is introduced to the body, leading to the development of immune memory, which ensures rapid and robust future protection if the real pathogen is encountered.

• Alternative wording: “substance prepared from the causative agent, its products, or a synthetic substitute acting as an antigen without inducing disease, but rather generating a protective immune response.”

• Immunity = a protective state conferred by the adaptive immune system, involving highly specific antibodies, memory B cells, and/or memory T cells, preventing or ameliorating disease upon re-exposure to a specific pathogen.

• Antigen = any molecular structure (e.g., proteins, polysaccharides, lipids) recognized by the highly specific adaptive immune receptors (B cell receptors / BCRs on B cells and T cell receptors / TCRs on T cells), capable of eliciting an immune response.

History of Vaccines

• 1500s (Asia): Variolation (also known as inoculation) was practiced, particularly in China and India. This involved deliberately exposing healthy individuals to smallpox by applying material from smallpox lesions (pus or scabs) to scratched skin or by insufflation.

  • The goal was to induce a milder, non-lethal form of the disease that would confer lifelong immunity.

• 1722: Lady Mary Wortley Montagu, having observed variolation in Turkey, introduced the practice to Europe.

  • Mixed success; while it reduced mortality compared to natural infection, it still carried a significant risk (approximately 1-2%) of inducing full-blown smallpox and death, and could also spread the disease.

• 1798: Edward Jenner, an English physician, pioneered modern vaccination.

  • Observed that milkmaids who had contracted mild cowpox (a related but less severe bovine disease) seemed to be protected from the deadly human smallpox.

  • Inoculated 8-year-old James Phipps with material from a cowpox lesion. After the boy recovered from mild cowpox, Jenner challenged him with smallpox material, but Phipps did not develop the disease.

  • This landmark experiment marked the conceptual birth of modern vaccination, demonstrating the principle of using a weakened or related (attenuated) virus to induce protective immunity against a more dangerous one, significantly reducing risk compared to variolation.

• 1980: The World Health Organization (WHO) officially declared smallpox eradicated globally, a monumental achievement made possible by widespread vaccination efforts.

  • Samples of the Variola virus are controversially retained in two high-security labs for research and potential biodefence purposes.

Virology Fundamentals

• “A piece of bad news wrapped in protein.” — Peter Medawar (Nobel laureate)

• Viruses = obligate intracellular parasites consisting of genetic code (either DNA or RNA) encased within a protective protein shell (capsid).

  • Naked virus: Composed solely of nucleic acid and a protein capsid.

  • Enveloped virus: Consists of the nucleocapsid (nucleic acid + capsid) surrounded by an outer lipid envelope, which is typically derived from the host cell membrane during budding.

• Capsid: The protein shell that encloses the viral genome, built from repeating protein subunits called capsomeres. The capsid protects the nucleic acid and facilitates attachment to host cells.

• Not alive: Viruses are not considered living organisms because they lack cellular machinery and cannot replicate or carry out metabolic processes independently. They absolutely require a living host cell to reproduce.

• Size scale (for context, viruses are much smaller than most cellular organelles):

  • Lysosome 100-1200nm (a cellular organelle)

  • Influenza 80-120nm (well-known human pathogen)

  • HIV 90{-}160nm (human immunodeficiency virus)

  • SARS-CoV-2 60-140nm (the virus causing COVID-19)

Virus Shapes / Families

• Helical (e.g.- Tobacco mosaic virus, Influenza virus): Capsomeres arranged in a spiral staircase or helix around the nucleic acid.

• Polyhedral / icosahedral (e.g.- Adenovirus, Herpesvirus): Capsid forms a symmetrical, multi-sided structure, often an icosahedron (20 faces, 12 vertices).

• Spherical (enveloped; e.g.- Influenza, SARS-CoV-2): These are typically icosahedral or helical nucleocapsids surrounded by a generally spherical lipid envelope, giving them a rounded appearance.

• Complex (e.g.- Bacteriophage: head–tail “spaceship”): Viruses with irregular or unique structures that do not fit into the simple helical or polyhedral categories, often possessing additional components like protein tails or complex outer walls.

Classification Criteria

Viruses are classified based on several key characteristics, most notably by the Baltimore classification system, which categorizes viruses based on their genome type and replication strategy:

• Nucleic acid type: All viruses contain either DNA or RNA as their genetic material.

  • DNA viruses (e.g., Herpesviruses, Adenoviruses) typically replicate in the host cell nucleus.

  • RNA viruses (e.g., Influenza, HIV, Coronaviruses) often replicate in the cytoplasm.

  • Sense: Refers to the polarity of the RNA genome.

    • + sense RNA (positive sense): The genome can directly serve as messenger RNA (mRNA) for protein synthesis.

    • - sense RNA (negative sense): The genome must first be transcribed into a + sense mRNA strand before protein synthesis can occur.

• Genome topology: Describes the physical shape of the viral genome.

  • Linear: The nucleic acid is a straight chain.

  • Circular: The nucleic acid forms a closed loop.

  • No double-stranded circular RNA viruses have been discovered to date.

• Strandedness: Refers to whether the nucleic acid is single-stranded (ss) or double-stranded (ds).

• Presence/absence of envelope: Distinguishes between naked and enveloped viruses, impacting their stability and susceptibility to certain disinfectants.

• Replication strategy & polymerase requirements: How the virus synthesizes its mRNA and replicates its genome, including whether it uses host or viral polymerases.

Coronaviruses & SARS-CoV-2 Specifics

• 7 known human coronaviruses, all belonging to the Coronaviridae family (named for their crown-like spikes on the surface).

  • 4 cause hickapprox 15{-}30 ext{%} of common winter colds (e.g., OC43, 229E, NL63, HKU1), typically causing mild upper respiratory infections.

  • 3 severe: The more pathogenic human coronaviruses responsible for serious respiratory illnesses include SARS-CoV-1 (Severe Acute Respiratory Syndrome, 2002-2003), MERS-CoV (Middle East Respiratory Syndrome, 2012), and SARS-CoV-2 (the causative agent of COVID-19, emerged in 2019).

• Nomenclature:

  • SARS = Severe Acute Respiratory Syndrome. This describes the clinical presentation of the disease, often characterized by acute lung injury.

  • CoV-2 = The second identified SARS coronavirus, distinguishing it from SARS-CoV-1.

  • COVID-19 = Corona Virus Disease identified in 2019.

• Genomic features:

  • Largest known RNA genomes (approximate size of 30 ext{ kb}), which is unusual for RNA viruses and contributes to their complexity.

  • Encodes for hickapprox 29 proteins, including 4 crucial structural proteins (Spike, Envelope, Membrane, and Nucleocapsid) that form the virion.

  • Possesses a lipid envelope, which makes the virus susceptible to disruption by detergents, alcohol-based hand sanitisers, and soap (hence the effectiveness of hand washing).

  • Contains a unique non-structural protein 14 (nsp14) with exonuclease proofreading activity. This enzyme corrects errors during RNA replication, leading to a much lower lethal mutation rate compared to other RNA viruses. However, it still allows for viable adaptive mutations, leading to the emergence of variants of concern (e.g., Alpha, Delta, Omicron).

SARS-CoV-2 Entry & Replication Cycle

• Spike (S) protein, located on the viral surface, is critical for host cell entry as it binds specifically to the Angiotensin-converting enzyme 2 (ACE2) receptor expressed on the host cell membrane, particularly abundant in lung, heart, and kidney cells.

• TMPRSS2 (transmembrane serine protease 2), a host cell protease, primes the Spike protein after binding. This cleavage event facilitates conformational changes in the Spike protein that are essential for membrane fusion, allowing the viral envelope to fuse with the host cell membrane or endosomal membrane, leading to endocytosis (where the entire virion or its nucleocapsid enters the cell).

• Once inside the host cell cytoplasm:

  • Translation of viral genomic RNA (specifically ORF1a/b) occurs, leading to the production of the viral replication–transcription complex (RTC), which includes viral RNA polymerases.

  • The RTC then synthesizes new genomic RNA ( ext{RNA}{genomic}) and multiple subgenomic RNAs ( ext{RNA}{subgenomic}).

  • ext{RNA}*{subgenomic} molecules serve as mRNA templates for the translation of structural proteins (S, E, M, N).

  • Assembly of new virions takes place in the ER–Golgi intermediate compartment (ERGIC) of the host cell, where newly synthesized structural proteins and genomic RNA are packaged.

  • Finally, newly assembled virions are released from the host cell via vesicular exocytosis (a process where vesicles containing virions fuse with the cell membrane), allowing them to infect new cells.

Immune System Overview

Three Lines of Defence: A Coordinated Biological Barrier

1. Physical & chemical barriers: The body’s first and immediate line of defence, preventing pathogen entry. Examples include the tough, acidic skin; mucus membranes lining respiratory and digestive tracts (trapping pathogens); cilia (sweeping pathogens clear); coughing and sneezing reflexes; tears and saliva (containing antimicrobial enzymes like lysozyme); and the acidic environment of the stomach.

2. Innate cellular & humoral immunity: A rapid, non-specific response that acts within minutes to hours. This includes phagocytic cells (like macrophages and neutrophils that engulf pathogens), natural killer (NK) cells, antimicrobial peptides, the complement system, and the inflammatory response (swelling, redness, pain due to increased blood flow and immune cell infiltration).

3. Adaptive immunity: A delayed but highly specific and memory-generating response, initiating days after exposure. It involves specialized white blood cells, B and T lymphocytes, which identify and target specific pathogens, leading to long-term protection.

Innate Immunity: The Immediate, Non-Specific Response

• Immediate: Responds within minutes to hours of pathogen exposure.

• Non-specific pattern recognition: Relies on Pattern Recognition Receptors (PRRs) on immune cells that recognize conserved molecular patterns unique to pathogens, known as Pathogen-Associated Molecular Patterns (PAMPs) (e.g., bacterial lipopolysaccharide, viral double-stranded RNA) or Damage-Associated Molecular Patterns (DAMPs) from damaged host cells.

• Key cells/molecules:

  • Neutrophils: Abundant phagocytes that are typically first responders to infection.

  • Macrophages: Larger phagocytic cells that engulf pathogens, present antigens, and release cytokines.

  • Dendritic cells (DCs): Professional antigen-presenting cells (APCs) that link innate and adaptive immunity by capturing antigens and migrating to lymph nodes to activate T cells.

  • NK cells (Natural Killer cells): Lymphocytes that identify and kill virus-infected cells and tumor cells without prior activation, by recognizing cells lacking MHC-I molecules or displaying stress markers.

  • Complement system: A cascade of plasma proteins that can directly lyse pathogens, opsonize them for phagocytosis, and promote inflammation.

  • Interferons: Antiviral proteins produced by infected cells that inhibit viral replication in neighboring cells and activate immune responses.

  • Cytokines: Small proteins that act as chemical messengers between immune cells, regulating immunity, inflammation, and hematopoiesis.

Adaptive Immunity: The Specific and Memory-Generating Response

• Delayed: Takes several days (typically 5-10 days) to mount a primary response upon first exposure to a pathogen.

• Highly specific: Each B or T lymphocyte recognizes only one specific epitope (a small unique part of an antigen). This precision allows for targeted attacks and minimizes collateral damage.

• Generates memory: Upon re-exposure to the same pathogen, memory B and T cells rapidly proliferate and differentiate, leading to a faster, stronger, and more effective secondary immune response that often prevents disease symptoms.

• Components:

  • Humoral immunity: Mediated by B lymphocytes which, upon activation (often with T helper cell help), differentiate into plasma cells that produce vast quantities of highly specific antibodies.

    • Antibodies: Soluble proteins that neutralize pathogens (e.g., block viral binding), opsonize them for phagocytosis, and activate the complement system.

  • Cell-mediated immunity: Mediated by T lymphocytes.

    • T helper (CD4⁺) cells: Crucial orchestrators of immune responses, secreting cytokines that help activate B cells, cytotoxic T cells, and other immune cells.

    • Cytotoxic T (CD8⁺) cells: Recognize and kill host cells that are infected with intracellular pathogens (like viruses) or are cancerous, by inducing apoptosis.

• Antigen processing and presentation:

  • Exogenous antigen: Antigens derived from outside the cell (e.g., engulfed bacteria, viral particles) are processed and presented on Major Histocompatibility Complex class II (MHC-II) molecules. This presentation primarily activates CD4⁺ T helper (Th) cells.

  • Endogenous (viral) antigen: Antigens synthesized within the cell (e.g., viral proteins produced during infection, tumor proteins) are processed and presented on Major Histocompatibility Complex class I (MHC-I) molecules. This presentation primarily activates CD8⁺ cytotoxic T (Tc) cells, leading to the destruction of the infected cell.

Cascade Example (Coronavirus Immune Response)

Let's trace a typical immune response to a viral infection like coronavirus:

1. Antigen Uptake and Processing: A dendritic cell (a type of professional APC) in the lung encounters and engulfs a SARS-CoV-2 virion. Inside the dendritic cell, the virion is digested into smaller peptide fragments, and a spike peptide (an exogenous antigen) is loaded onto MHC-II molecules.

2. Migration and T Cell Activation: The dendritic cell, now activated and carrying the antigen, migrates through lymphatic vessels to a draining lymph node. In the lymph node, it presents the spike peptide on MHC-II to naive CD4⁺ T helper (Th) cells, activating specific Th cells.

3. Cytokine Secretion and B Cell/Tc Activation: The activated Th cells proliferate and differentiate, secreting a variety of cytokines (e.g., interleukins). These cytokines play crucial roles:

   • They help activate specific B cells (which have also encountered the viral antigen) to undergo class-switch recombination (producing different antibody types like IgG, IgA) and affinity maturation (producing antibodies with higher binding strength), leading to differentiation into antibody-secreting plasma cells.

   • They promote the proliferation and differentiation of specific CD8⁺ cytotoxic T cells (which recognize internally produced viral antigens presented on MHC-I by infected cells) to eliminate ACE2⁺ infected cells.

• Memory Formation: After the infection is cleared, a subset of activated B and T cells survive and differentiate into long-lived memory B and T cells. These memory cells persist in the body for extended periods, forming the basis of long-term protection provided by both natural infection and vaccination.

Vaccine Composition (What’s in the Vial?)

Vaccines are carefully formulated products containing several key components beyond just the antigen:

• Antigen (core stimulant): The specific component derived from the pathogen (e.g., inactivated virus, viral protein, mRNA encoding a protein) that stimulates the immune system to produce a protective response. It is the active ingredient.

• Adjuvant (danger signal): A substance added to enhance the immune response to the antigen. Adjuvants typically work by creating a local inflammatory response, acting as a depot for the antigen, or stimulating innate immune receptors (like PRRs). Examples include aluminum salts ( ext{Al}^{3+} salts), which are common in many traditional vaccines. Adjuvants make the vaccine more potent and can reduce the amount of antigen needed.

• Preservatives (prevent microbial growth): Chemicals added in small amounts to prevent the growth of bacteria or fungi that could contaminate the vaccine vial after the seal is broken. Thiomersal (a mercury-containing organic compound) and 2-phenoxyethanol are examples, though thiomersal use has largely declined in many vaccines due to public concern (despite extensive safety data).

• Stabiliser (maintain conformational integrity): Substances that protect the vaccine antigens from degradation due to temperature fluctuations, freezing, or agitation during storage and transport. They ensure the antigen retains its three-dimensional structure, which is crucial for immune recognition. Gelatin and sugars like sucrose or lactose are common stabilizers.

• Surfactant (prevent aggregation): Agents like polysorbate 80 or Triton X-100 are added to prevent the antigen and other components from sticking together (aggregating) or to the walls of the vaccine vial. This ensures the vaccine remains a homogenous solution and is properly delivered.

• Residuals from manufacturing: Trace amounts of substances used during the manufacturing process may remain, such as egg proteins (from egg-based vaccine production), formaldehyde (used to inactivate viruses), antibiotics (to prevent bacterial contamination during cell culture), or cell-culture media components. These are present in extremely small, safe quantities.

• Diluent (usually sterile ext{H} ext{*}2 ext{O}): A liquid used to dilute the vaccine to the correct concentration for administration, often sterile water for injection or saline solution, especially for lyophilized (freeze-dried) vaccines that need reconstitution.

Major Vaccine Types & Mechanisms

Mechanistic Flow (Generic Steps of Vaccine Action)

1. Vaccine Introduction: The vaccine is typically administered via intramuscular (IM) or subcutaneous (SC) injection, allowing components to reach local tissues and lymphatic drainage.

2. Uptake by APCs: Antigen-presenting cells (APCs), such as dendritic cells and macrophages, located at the injection site, engulf the vaccine antigens.

3. Processing & MHC Presentation: APCs process the antigens and display critical fragments on their cell surface via MHC-I and MHC-II molecules. This is the crucial step for initiating the adaptive immune response.

4. Th Activation: The APCs migrate to regional lymph nodes, where they present the antigen-MHC complexes to specific naive T helper (CD4⁺) cells, which become activated and proliferate.

5. B cell & Tc Activation: Activated T helper cells, through cytokine release and direct cell-to-cell contact, help activate specific B cells (leading to their differentiation into plasma cells and antibody production) and specific cytotoxic T (CD8⁺) cells (which go on to kill infected cells).

6. Formation of Long-Lived Memory Cells: A subset of activated B cells and T cells differentiate into long-lived memory B cells and memory T cells, which can quickly respond upon subsequent exposure to the actual pathogen, providing protective immunity.

Vaccine Development Pipeline (Traditional vs Pandemic Fast-Track)

Vaccine development is a rigorous, multi-stage process, traditionally taking 10-15 years. The COVID-19 pandemic necessitated an unprecedented acceleration, leveraging existing technologies and increased funding to fast-track development.

1. Discovery / Exploratory Phase: This foundational stage involves basic research on the pathogen (genomics, structure, virulence factors), identification of potential antigens, and initial proof-of-concept studies. This phase can take 2-4 years.

2. Pre-clinical Phase (Laboratory & Animal Models): Promising vaccine candidates are tested in vitro (cell cultures) and in vivo (animal models like mice, ferrets, non-human primates). This phase, typically lasting 1-2 years, aims to:

   • Evaluate immunogenicity: Does the vaccine stimulate an immune response?

   • Assess protection: Does it protect animals from disease?

   • Determine basic safety and toxicity: Are there any immediate adverse effects?

3. Phase I Clinical Trials (Human Safety): The first administration to humans, involving a small group of 10–100 healthy adult volunteers (typically 18-55 years old), usually low-risk individuals. This phase focuses primarily on:

   • Safety: Are there unacceptable side effects?

   • Dose-finding: What are the optimal and safe dose ranges?

   • Preliminary immunogenicity: Does it induce an immune response in humans?

4. Phase II Clinical Trials (Expanded Safety & Immunogenicity): Involves a larger group of 100–500 diverse volunteers, including different age groups (e.g., elderly, children) and sometimes individuals with specific comorbidities, to simulate real-world populations. This phase expands on:

   • Extended safety: Further evaluation of adverse events across a broader demographic.

   • Immune correlates: Measuring the strength and type of immune response (e.g., antibody levels, T cell responses) across various age and comorbidity groups.

5. Phase III Clinical Trials (Efficacy): The most extensive and critical phase, involving thousands (1,000s to tens of thousands) of participants. These are typically large-scale, randomised, placebo-controlled efficacy trials. The primary goal is to determine if the vaccine prevents the disease in a real-world setting.

   • Primary endpoint: The key measure is the reduction in disease incidence in the vaccinated group compared to the placebo group, expressed as Vaccine Efficacy (VE).

   • ext{VE} = 1 - rac{ ext{attack rate}{ ext{vacc}}}{ ext{attack rate}{ ext{placebo}}} where attack rate is the proportion of people who get the disease in each group.

   • Also monitors rare side effects that might not appear in smaller trials.

6. Regulatory Review: If Phase III trials demonstrate sufficient safety and efficacy, the vaccine manufacturer submits a comprehensive application to regulatory bodies (e.g., FDA in USA, TGA in Australia). These agencies meticulously review all pre-clinical and clinical data to decide on authorization or approval.

7. Phase IV / Post-marketing Surveillance: Even after approval, continuous monitoring of the vaccine's safety and effectiveness in the general population continues. This pharmacovigilance is crucial to:

   • Detect rare adverse events that may only appear once millions of doses are administered.

   • Monitor long-term efficacy and duration of protection.

   • Evaluate effectiveness in different populations not fully represented in trials.

COVID-19 Numerical Snapshot (WHO 2023 - indicative numbers reflect rapid development)

• hickapprox 199 candidates in pre-clinical development

• hickapprox 54 candidates in Phase I

• hickapprox 44 candidates in Phase I/II combined

• hickapprox 15 candidates in Phase II

• hickapprox 18 candidates in Phase II/III

• hickapprox 50 candidates in Phase III (some may have since advanced or been discontinued)

• hickapprox 11 vaccines authorized/approved for public use (with ongoing Phase IV surveillance)

Practical, Ethical & Regulatory Considerations

• Funding & international collaboration: The unprecedented scale of government funding, private investment, and global scientific collaboration significantly accelerated COVID vaccine timelines. This was a major factor in overcoming traditional bottlenecks like funding acquisition and sequential phase progression.

• SARS-CoV-2 proofreading & variants: The presence of exonuclease proofreading in SARS-CoV-2 significantly reduces the rate of lethal mutations during replication, making the virus more stable than many other RNA viruses. However, it still allows for the accumulation of advantageous (adaptive) mutations that confer survival benefits, leading to the emergence of multiple variants (e.g., Delta rapidly outcompeted Alpha, then Omicron outcompeted Delta), which can impact vaccine effectiveness.

• Regulatory bodies: Key agencies responsible for vaccine approval globally are:

  • USA: FDA (Food and Drug Administration), specifically its Center for Biologics Evaluation and Research (CBER).

  • Australia: TGA (Therapeutic Goods Administration), which operates under the Department of Health. ✱ (Answer to student query)

• Egg-based vs. cell-culture manufacturing: Early viral vaccines (like many influenza vaccines) were traditionally cultivated in embryonated chicken eggs. Modern platforms increasingly utilize cell-culture lines (e.g., Vero cells, CHO cells, HEK293 cells). This shift can have implications for manufacturing speed, scalability, and suitability for individuals with severe egg allergies.

• Biodefence & ethical debate: The maintenance of eradicated pathogens, such as smallpox stockpiles, presents an ongoing ethical dilemma. While proponents argue for their scientific value for research and biodefence preparedness, opponents highlight the immense bioterrorism risk and the potential for accidental release.

• Vaccine hesitancy: Misinformation and distrust, leading to vaccine hesitancy, continue to threaten global eradication goals for preventable diseases (e.g., measles resurgence in several countries where vaccination rates have declined). This undermines herd immunity.

Q&A Highlights from Lecture

• Why multiple COVID variants despite proofreading?

  • Proofreading limits catastrophic errors that would kill the virus, allowing it to survive high replication rates. However, it does not prevent all mutations and specifically allows for adaptive mutations that confer a selective advantage (e.g., increased transmissibility, immune escape), leading to variants.

• Culture methods:

  • Embryonated eggs are common for influenza vaccine production due to established infrastructure and high yield.

  • Other common cell lines widely used in vaccine manufacturing include Vero cells (African green monkey kidney cells), CHO cells (Chinese hamster ovary cells), and HEK293 cells (human embryonic kidney cells).

• Australian approval agency:

  • The Therapeutic Goods Administration (TGA) is the regulatory body responsible for approving vaccines and other medicines in Australia.

• Near-eradication target:

  • Polio (wild-type cases <100 worldwide as of recent years) is on the verge of eradication but is hindered by geopolitical instability, conflict zones, and ongoing vaccine uptake issues in remaining endemic regions.

Numbers, Statistics & Equations

• Typical virion diameters listed earlier (Lysosome hickapprox 100{-}1200 ext{ nm}, Influenza hickapprox 80{-}100 ext{ nm}, HIV hickapprox 90{-}160 ext{ nm}, SARS-CoV-2 hickapprox 60{-}140 ext{ nm}).

• Vaccine efficacy formula: VE = 1 - rac{ ext{attack rate}{ ext{vacc}}}{ ext{attack rate}{ ext{placebo}}} (used in Phase III clinical trials).

• Three-line defence timeline: Innate immunity (minutes–hours) vs. Adaptive immunity (days).

• Historical dates: 1722 (Lady Mary Wortley Montagu introduces variolation to Europe); 1798 (Edward Jenner's smallpox vaccination); 1980 (Smallpox officially eradicated); 2019 (SARS-CoV-2 emergence and recognition of COVID-19).

Connections & Real-World Relevance

• Computer vs. biological viruses analogy: Both consist of a genetic code (program) and a protective shell. Both exploit host machinery (computer or cell) to replicate and spread, and both can be